1. Field of the Invention
The present invention relates to a direct-injection spark-ignition engine, and more particularly to a direct-injection spark-ignition engine designed to promote catalyst activation during cold engine operation.
2. Description of the Background Art
Heretofore, with a view to achieving improvement in fuel economy, a direct-injection spark-ignition engine operable to directly inject fuel into each cylinder has been employed. In such a direct-injection spark-ignition engine, a compression ratio can be set at a relatively high value to enhance engine efficiency, and a fuel injection amount can be freely adjusted regardless of an intake air amount.
Late years, in connection with tighter exhaust emission regulations, there has been an increasing need for achieving earlier catalyst activation. The earlier catalyst activation requires quickly raising an exhaust gas temperature. As means to raise an exhaust gas temperature, it is contemplated to maximally retard an ignition timing.
However, a large amount of ignition timing retard causes deterioration in combustion stability. Thus, it has been expected to provide a technique of retarding the ignition timing while ensuring combustion stability.
In this connection, JP 2006-250050A (hereinafter referred to as Patent Document 1) proposes a control system for a direct-injection spark-ignition engine, wherein a fuel injection period is divided into a main injection period for injecting fuel at a first timing set in a range across a top dead center of a compression stroke (compression TDC) and an early injection period for injecting fuel at a second timing earlier than the first timing (e.g., during an intake stroke), and combustion is produced under a condition that an ignition timing is largely retarded relative to the compression TDC.
In this engine control system, fuel is preliminarily injected at an early timing (e.g., during an intake stroke) to allow the injected fuel to be spread over a combustion chamber in each cylinder before the fuel injection in the main injection period, so that combustion initiated by igniting a fuel spray in the main injection period will progress in the combustion chamber at a relatively slow speed. This makes it possible to stably produce combustion.
In other words, the engine control system is designed to change a state of an air-fuel mixture in each cylinder from a so-called “stratified mode” where a rich air-fuel mixture exists around a spark plug and only air exists around the rich air-fuel mixture, to a so-called “weak-stratified mode” where a lean air-fuel mixture exists around the rich air-fuel mixture.
As disclosed in the Patent Document 1, it is contemplated to employ a multi-hole injector as a fuel injector for a direct-injection spark-ignition engine. A direct-injection spark-ignition engine using the multi-hole injector is also disclosed, for example, in JP 2007-092693A (hereinafter referred to as Patent Document 2).
In the engine disclosed in the Patent Document 2, a fuel spray is introduced into a concave portion provided in a piston crown surface to enhance robustness of stratified combustion and facilitate vaporization/atomization in a homogenous combustion region.
JP 2007-154827A (hereinafter referred to as Patent Document 3) discloses an engine, wherein a concave portion in a piston crown surface is formed to have a spherically curved sub-surface with a center at an ignition point of a spark plug. The concave portion having a spherically curved sub-surface can define a generally spherical-shaped combustion chamber, so that a homogenous and excellent combustion state can be constantly maintained.
In the engine control system disclosed in the Patent Document 1, the fuel injection in the main injection period has to be performed at the first timing set in the range across a compression TDC at which an in-cylinder pressure is maximized. This gives rise to a need for setting an injection pressure of a fuel injector at an extremely high value. Moreover, fuel is injected toward a spark plug, and thereby combustion has to be initiated under a condition that an ignition timing of the spark plug is accurately coincident with a timing when the injected high-pressure fuel spray reaches the spark plug.
However, in the operation of injecting fuel from the fuel injector toward the spark plug at a high pressure, the management of an fuel injection timing becomes more critical. Specifically, if the fuel injection timing deviates from an adequate point even slightly, a problem of deterioration in combustion state will occur. Particularly, during cold engine operation, it is difficult to reliably keep the fuel injection timing constant, due to high fuel viscosity.
Further, if the fuel injector is arranged to inject fuel toward the spark plug as in the Patent Document 1, fuel cannot be injected toward a lower region of the combustion chamber in a homogenous combustion mode during normal (warmed-up) engine operation, to cause a problem of deterioration in combustion efficiency during the homogenous combustion mode.
The engine disclosed in the Patent Document 2 is not originally intended to provide measures for early catalyst activation during cold engine operation, and thereby cannot solve the problems in the Patent Document 1. Similarly, the engine disclosed in the Patent Document 3 is not originally intended to provide measures for early catalyst activation during cold engine operation, and thereby cannot solve the problems in the Patent Document 1.